There is known an annular air-hammer drilling apparatus (cf. FRG Pat. No. 2,854,461 IPC E21C 3/24, 1978) termed perforator, which incorporates a rock-cutting tool and an annular hammer contained in a cylindrical case with air-distributing ports. The perforator is provided with a check valve and an internal chips offtake pipe and is employed with a concentric drill string. Featuring complex design and light-gauge shapes, the perforator, however, lacks operational reliability and, therefore, fails to find wide industrial application.
There is also known an annular air-hammer drilling apparatus (cf. USSR Inventor's Certificate No. 1,133,388, IPC E21C 3/24, 1985), comprising a shell accommodating a chips-receiving sleeve, a hollow cylindrical case with inlet and outlet ports, all these fitted co-axially, the case carrying a ring-shaped hammer capable of reciprocating back and forth and forming forward- and back-stroke chambers with the case. The lower part of the shell accommodates a rock-cutting tool capable of moving axially with an axial opening and at least a single blow-off passage permanently connected with the air distribution system and with a bottom hole. Interposed between the hammer and the case is a sleeve with provision for axial displacement, which has an annular recess at its midlength fitted whereinto is a projecting stop of the hammer.
The above design features permit control of the time interval during which the compressed gaseous fluid is being fed into the working chambers and, consequently, increase the impact power of the apparatus. However, in the aforesaid device, spent air outflows from the working chambers directly into the chips-receiving sleeve by-passing the bottom hole. For this reason, due to the insufficient bottom-hole cleaning from cuttings, the axial opening of the rock-cutting tool and the chips-receiving sleeve are likely to be plugged up.
All in all, these factors affect drilling efficiency in permafrost.